It is sometimes useful to measure physical parameters within the brain. It is also sometimes useful to deliver energy to structures within the brain.
A difficulty arises from the presence of a skull. Although the skull serves useful functions, it hampers the ability to make measurements of activity within the brain and to deliver energy therein.
Of course, it is always possible to make holes in the skull and insert various devices through those holes. However, this is a somewhat invasive procedure that carries certain risks.
It is also possible to place electrodes on the scalp. However, there are quite a few layers between the scalp and the brain. Thus, a signal of interest must pass through many layers before reaching the electrodes. This results in considerable attenuation.
Additionally, stray electrical signals from nerves that actuate muscles in the vicinity of the scalp and large signals from contracting muscle fibers themselves can interfere with the signal of interest.
To make matters worse, the scalp is not completely stationary relative to the skull. Thus, when an electrode is attached to the scalp, it can move relative to the brain. These movements cause artifacts in the measured signal and impede correct accurate registration. All of this combines to result in an often unacceptably low signal-to-noise ratio for measurements and in inaccurate delivery of energy when attempting to effect a stimulus.
In a similar fashion, it would be advantageous to place other types of optical or sonic stimulators/transducers such as those measuring oxygen saturation closer to the brain without having to be limited by traversing the skull, muscle, and scalp overlying the brain.